The invention is related to a process for producing an absorbent polymer, an absorbent polymer obtainable by this process, an absorbent polymer, a composite, a process for producing the composite, a composite obtainable by this process, chemical products as well as the use of the absorbent polymer and of the composite.
In order to form so-called “superabsorbent” polymers a polymerization of different types of normally water-soluble monomers, often however also of water insoluble co-monomers, together with the presence of cross-linkers, is necessary. The addition of the cross-linkers occurs during or after the polymerization. Superabsorbent polymers of this type are lightly cross-linked, water insoluble hydrogel polymers, which have a large capacity for water absorption in the dry and in the essentially water-free state. The absorbed quantities of water can constitute a multiple of the weight of the superabsorbent polymer itself.
Because of this high absorption capacity superabsorbent polymers are suitable for water absorbent structures and objects, such as for example baby diapers, incontinence products or sanitary napkins.
Although the ratio of the absorbed weight of water to the dry weight of the polymer, i.e. the absorption capacity, is sufficient for applications of this type, the rate with which the water is absorbed, i.e. the rate of absorption, is limited and for many application cases unsatisfactory. In many cases the high absorption capacity of the superabsorbent polymers has no effect, since the slow absorption rate prevents a sufficiently fast absorption of the water and thereby renders difficult the use of the polymers in hygiene applications.
WO-A-98/49221 describes a process for improving the properties of absorbent polymer particles, in which the polymer particles which, for example during a secondary cross-linking reaction, have been heated beforehand for at least 10 minutes at least 170° C., are mixed with an aqueous additive solution. Upon mixing the polymers with an aqueous solution, however, agglomeration of the polymer particles occurs, which according to the teaching of WO-A-98/49221 can only be prevented by addition of additives to the aqueous solution, for example in the form of singly or multiply charged metal cations.
In general the object of the present invention is to overcome the disadvantages arising from the state of the art.
It is in particular an object of the present invention to specify a process by which the superabsorbent polymers can be produced, which polymers have an increased rate of absorption towards water without appreciably reducing their absorption capacity. In addition this process should be carried out in a simple way and be possible without the use of additives, in particular without the use of organic additives.
A further object of the invention consists especially in providing a superabsorbent polymer, composites which comprise superabsorbent polymers of this type and chemical products which comprise superabsorbent polymers or composites of these types, wherein the superabsorbent polymer has an increased rate of absorption towards water. In addition the absorption properties of these polymers under mechanical load, which occur in particular upon transport of the polymer particles in conveyor systems, should not be negatively influenced. In particular the absorption capacity of the polymer under a force load due to mechanical load should not be decreased at all if possible or at least be only slightly reduced (=mechanical stability).